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3. Design by Contract
Design by Contract
© Oscar Nierstrasz
Design by Contract
2.2
Bertrand Meyer, Object-Oriented Software Construction, Prentice Hall, 1997.
© Oscar Nierstrasz 3
Roadmap
> Contracts> Stacks> Design by Contract> A Stack Abstraction> Assertions> Example: balancing parentheses
Safety Patterns
© Oscar Nierstrasz 4
Roadmap
> Contracts> Stacks> Design by Contract> A Stack Abstraction> Assertions> Example: balancing parentheses
Safety Patterns
Class Invariants
© Oscar Nierstrasz
Design by Contract
5
An invariant is a predicate that must hold at certain pointsin the execution of a program
A class invariant characterizes the valid states of instancesIt must hold:
1. after construction2. before and after every public method
© Oscar Nierstrasz
Design by Contract
2.6
Contracts
A contract binds the client to pose valid requests, andbinds the provider to correctly provide the service.
© Oscar Nierstrasz
Design by Contract
2.7
Contract violations
If either the client or the provider violates the contract, an exception is raised.
NB: The service does not need to implement any special logic to handle errors — it simply raises an exception!
© Oscar Nierstrasz
Design by Contract
2.8
Exceptions, failures and defects
> An exception is the occurrence of an abnormal condition during the execution of a software element.
> A failure is the inability of a software element to satisfy its purpose.
> A defect (AKA “bug”) is the presence in the software of some element not satisfying its specification.
Disciplined Exceptions
> There are only two reasonable ways to react to an exception:1. clean up the environment and report failure to the client
(“organized panic”)2. attempt to change the conditions that led to failure and retry
© Oscar Nierstrasz
Design by Contract
2.9
A failed assertion often indicates presence of a software defect, so “organized panic” is usually the best policy.
© Oscar Nierstrasz 10
Roadmap
> Contracts> Stacks> Design by Contract> A Stack Abstraction> Assertions> Example: balancing parentheses
Safety Patterns
© Oscar Nierstrasz
Design by Contract
2.11
Stacks
A Stack is a classical data abstraction with many applications in computer programming.Stacks support two mutating methods: push and pop.
Operation Stack isEmpty() size() top()
TRUE 0 (error)
push(6) FALSE 1 6
push(7) FALSE 2 7
push(3) FALSE 3 3
pop() FALSE 2 7
push(2) FALSE 3 2
pop() FALSE 2 7
6 7
6 7 3
6 7
6 7 2
6 7
6
© Oscar Nierstrasz
Design by Contract
2.12
Stack pre- and postconditions
Stacks should respect the following contract:
service pre post
isEmpty() - no state change
size() - no state change
push(Object item) -not empty, size == old size + 1,top == item
top() not empty no state change
pop() not empty size == old size -1
Stack invariant
> The only thing we can say about the Stack class invariant is that the size is always ≥ 0— we don’t know anything yet about its state!
© Oscar Nierstrasz
Design by Contract
13
© Oscar Nierstrasz 14
Roadmap
> Contracts> Stacks> Design by Contract> A Stack Abstraction> Assertions> Example: balancing parentheses
Safety Patterns
Design by Contract
© Oscar Nierstrasz
Design by Contract
2.15
“If you promise to call S with the precondition satisfied, then I, in return, promise to deliver a final state in which the post-condition is satisfied.”
Consequence:—if the precondition does not hold, the object is not required to
provide anything! (in practice, an exception is raised)
When you design a class, each service S provided must specify a clear contract.
© Oscar Nierstrasz
Design by Contract
2.16
In other words …
Design by Contract = Don’t accept anybody else’s garbage!
© Oscar Nierstrasz
Design by Contract
2.17
Pre- and Post-conditions
The pre-condition binds clients: — it defines what the data abstraction requires for a call to the
operation to be legitimate— it may involve initial state and arguments— example: stack is not empty
The post-condition, in return, binds the provider: — it defines the conditions that the data abstraction ensures on
return— it may only involve the initial and final states, the arguments and
the result— example: size = old size + 1
© Oscar Nierstrasz
Design by Contract
2.18
Benefits and Obligations
A contract provides benefits and obligations for both clients and providers:
Obligations Benefits
Client Only call pop() on a non-empty stack!
Stack size decreases by 1.Top element is removed.
ProviderDecrement the size. Remove the top element.
No need to handle case when stack is empty!
© Oscar Nierstrasz 19
Roadmap
> Contracts> Stacks> Design by Contract> A Stack Abstraction> Assertions> Example: balancing parentheses
Safety Patterns
© Oscar Nierstrasz
Design by Contract
2.20
StackInterface
Interfaces let us abstract from concrete implementations:
How can clients accept multiple implementations of a data abstraction?
Make them depend only on an interface or an abstract class.
public interface StackInterface {public boolean isEmpty();public int size();public void push(Object item);public Object top() ;public void pop();
}
© Oscar Nierstrasz
Design by Contract
2.21
Interfaces in Java
Interfaces reduce coupling between objects and their clients:
> A class can implement multiple interfaces— ... but can only extend one parent class
> Clients should depend on an interface, not an implementation— ... so implementations don’t need to extend a specific class
Define an interface for any data abstraction that will have more than one implementation
© Oscar Nierstrasz
Design by Contract
2.22
Stacks as Linked Lists
A Stack can easily be implemented by a linked data structure:
stack = new Stack();stack.push(6);stack.push(7);stack.push(3);stack.pop();
© Oscar Nierstrasz
Design by Contract
2.23
LinkStack Cells
We can define the Cells of the linked list as an inner class within LinkStack:
public class LinkStack implements StackInterface {private Cell top;private class Cell {
Object item;Cell next;Cell(Object item, Cell next) {
this.item = item;this.next = next;
}}...
}
© Oscar Nierstrasz
Design by Contract
2.24
Private vs Public instance variables
When should instance variables be public? Always make instance variables private or protected.
The Cell class is a special case, since its instances are strictly private to LinkStack!
© Oscar Nierstrasz
Design by Contract
2.25
LinkStack abstraction
The constructor must construct a valid initial state:
public class LinkStack implements StackInterface {...private int size;public LinkStack() {
// Establishes the class invariant.top = null;size = 0;
}...
© Oscar Nierstrasz
Design by Contract
2.26
Class Invariants
A class invariant is any condition that expresses the valid states for objects of that class:
> it must be established by every constructor> every public method
— may assume it holds when the method starts— must re-establish it when it finishes
Stack instances must satisfy the following invariant:> size ≥ 0> ...
© Oscar Nierstrasz
Design by Contract
2.27
LinkStack Class Invariant
A valid LinkStack instance has an integer size, and a top that points to a sequence of linked Cells, such that:— size is always ≥ 0— When size is zero, top points nowhere (== null)— When size > 0, top points to a Cell containing the top item
When to check invariants?
> In principle, check invariants:— at the end of each constructor— at the end of every public mutator
© Oscar Nierstrasz
Design by Contract
28
© Oscar Nierstrasz 29
Roadmap
> Contracts> Stacks> Design by Contract> A Stack Abstraction> Assertions> Example: balancing parentheses
Safety Patterns
© Oscar Nierstrasz
Design by Contract
2.30
Assertions
> An assertion is a declaration of a boolean expression that the programmer believes must hold at some point in a program. — Assertions should not affect the logic of the program— If an assertion fails, an exception is raised
x = y*y;assert x >= 0;
© Oscar Nierstrasz
Design by Contract
2.31
Assertions
Assertions have four principle applications:1. Help in writing correct software
— formalizing invariants, and pre- and post-conditions
2. Documentation aid— specifying contracts
3. Debugging tool— testing assertions at run-time
4. Support for software fault tolerance— detecting and handling failures at run-time
© Oscar Nierstrasz
Design by Contract
2.32
Assertions in Java
assert is a keyword in Java since version 1.4
will raise an AssertionError if expression is false.— NB: Throwable Exceptions must be declared; Errors need not
be!
Be sure to enable exceptions in eclipse! (And set the vm flag -enableassertions [-ea])
assert expression;
© Oscar Nierstrasz
Design by Contract
2.33
Enabling assertions in eclipse
© Oscar Nierstrasz
Design by Contract
2.34
Checking pre-conditions
Assert pre-conditions to inform clients when they violate the contract.
When should you check pre-conditions to methods? Always check pre-conditions, raising exceptions if they
fail.
public Object top() {assert(!this.isEmpty()); // pre-conditionreturn top.item;
}
NB: This is all you have to do!
© Oscar Nierstrasz
Design by Contract
2.35
Checking class invariants
Every class has its own invariant:
protected boolean invariant() {return (size >= 0) &&
( (size == 0 && this.top == null)|| (size > 0 && this.top != null));
}
Why protected and not private?
© Oscar Nierstrasz
Design by Contract
2.36
Checking post-conditions
Assert post-conditions and invariants to inform yourself when you violate the contract.
When should you check post-conditions? Check them whenever the implementation is non-trivial.
public void push(Object item) {top = new Cell(item, top);size++;assert !this.isEmpty(); // post-conditionassert this.top() == item; // post-conditionassert invariant();
}NB: This is all you have to do!
© Oscar Nierstrasz 37
Roadmap
> Contracts> Stacks> Design by Contract> A Stack Abstraction> Assertions> Example: balancing parentheses
Safety Patterns
© Oscar Nierstrasz
Design by Contract
2.38
Example: Balancing Parentheses
Problem:> Determine whether an expression containing
parentheses ( ), brackets [ ] and braces { } is correctly balanced.
Examples:> balanced:
> not balanced:
if (a.b()) { c[d].e(); }else { f[g][h].i(); }
((a+b())
© Oscar Nierstrasz
Design by Contract
2.39
A simple algorithm
Approach: > when you read a left parenthesis, push the matching
parenthesis on a stack> when you read a right parenthesis, compare it to the
value on top of the stack— if they match, you pop and continue— if they mismatch, the expression is not balanced
> if the stack is empty at the end, the whole expression is balanced, otherwise not
© Oscar Nierstrasz
Design by Contract
2.40
Using a Stack to match parentheses
Sample input: “( [ { } ] ]”
Input Case Op Stack
( left push ) )
[ left push ] )]
{ left push } )]}
} match pop )]
] match pop )
] mismatch ^false )
© Oscar Nierstrasz
Design by Contract
2.41
The ParenMatch class
A ParenMatch object uses a stack to check if parentheses in a text String are balanced:
public class ParenMatch {private String line;private StackInterface stack;
public ParenMatch (String aLine, StackInterface aStack){
line = aLine;stack = aStack;
}
© Oscar Nierstrasz
Design by Contract
2.42
A declarative algorithm
We implement our algorithm at a high level of abstraction:
public boolean parenMatch() {for (int i=0; i<line.length(); i++) {
char c = line.charAt(i);if (isLeftParen(c)) { // expect matching right paren later
stack.push(matchingRightParen(c)); // Autoboxed to Character} else {
if (isRightParen(c)) {// empty stack => missing left parenif (stack.isEmpty()) { return false; }if (stack.top().equals(c)) { // Autoboxed
stack.pop();} else { return false; } // mismatched paren
}}
}return stack.isEmpty(); // not empty => missing right paren
}
© Oscar Nierstrasz
Design by Contract
2.43
Ugly, procedural version
public boolean parenMatch() {char[] chars = new char[1000]; // ugly magic numberint pos = 0;for (int i=0; i<line.length(); i++) {
char c = line.charAt(i);switch (c) { // what is going on here?case '{' : chars[pos++] = '}'; break;case '(' : chars[pos++] = ')'; break;case '[' : chars[pos++] = ']'; break;case ']' : case ')' : case '}' :
if (pos == 0) { return false; }if (chars[pos-1] == c) { pos--; }else { return false; }break;
default : break;}
}return pos == 0; // what is this?
}
© Oscar Nierstrasz
Design by Contract
2.44
Helper methods
The helper methods are trivial to implement, and their details only get in the way of the main algorithm.
private boolean isLeftParen(char c) {return (c == '(') || (c == '[') || (c == '{');
}
private boolean isRightParen(char c) {return (c == ')') || (c == ']') || (c == '}');
}
© Oscar Nierstrasz
Design by Contract
2.45
Running parenMatch
public static void parenTestLoop(StackInterface stack) {BufferedReader in =
new BufferedReader(new InputStreamReader(System.in));String line;try {
System.out.println("Please enter parenthesized expressions to test");System.out.println("(empty line to stop)");do {
line = in.readLine();System.out.println(new ParenMatch(line, stack).reportMatch());
} while(line != null && line.length() > 0);System.out.println("bye!");
} catch (IOException err) {} catch (AssertionException err) {
err.printStackTrace();}
}
© Oscar Nierstrasz
Design by Contract
2.46
Running ParenMatch.main ...
Which contract has been violated?
Please enter parenthesized expressions to test(empty line to stop)(hello) (world)"(hello) (world)" is balanced()"()" is balancedstatic public void main(String args[]) {"static public void main(String args[]) {" is not balanced()"()" is not balanced}"}" is balanced
"" is balancedbye!
© Oscar Nierstrasz
Design by Contract
2.47
What you should know!
What is an abstract data type? What is the difference between encapsulation and
information hiding? How are contracts formalized by pre- and post-
conditions? What is a class invariant and how can it be specified? What are assertions useful for? What situations may cause an exception to be raised? How can helper methods make an implementation more
declarative?
© Oscar Nierstrasz
Design by Contract
2.48
Can you answer these questions?
When should you call super() in a constructor? When should you use an inner class? What happens when you pop() an empty java.util.Stack?
Is this good or bad? What impact do assertions have on performance? Can you implement the missing LinkStack methods?
License
© Oscar Nierstrasz 49
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Design by Contract
